The monitoring of intracellular oxidative metabolism, pH and PO2 will be carried out in: 1) excised rabbit papillary muscles, 2) ventricular strips of the toad, 3) the intact dog heart in situ. The effect of hypoxia-anoxia will be studied in these preparations and correlated with the excitability and mechanical properties of the tissue. In the last preparation, more over, these effects will be compared with those of ischemia. The near-infrared absorption band of cytochrome a,a3 will be studied in situ during hypoxia, anoxia and changes in myocardial function in 4) the intact cat heart and 5) the intact dog heart.